Thermoplastics are increasingly displacing metals in many tribological materials such as radial and axial bearings, engines, gears, seals rings, which are used in many automotive and industrial applications, especially for example in automobiles having 7 to 8 gears, which require materials having the strength and wear resistance found in lubricated metals. Internally lubricated polymers are replacing metals in these applications because of their ease of fabrication, higher performance, lower or little dependence on external lubrication, and lower overall cost.
PAEK polymers, including in particular poly(etheretherketone) (PEEK), poly(etherketone) (PEK) and poly(etherketoneketone) (PEKK) polymers are well known for their exceptional balance of technical properties, namely having excellent heat resistance, mechanical properties and wear resistance. However, when PAEK is used alone, the coefficient of friction is high, so that a resin having a low coefficient of friction such as notably polytetrafluoroethylene polymers (PTFE) or the like is added to the PAEK polymer to lower the coefficient of friction thereof.
PTFE polymers, while known for their thermal stability, high melting temperature, chemical inertness and lubricity and in particular having a low coefficient of friction, it is also known that their addition results in deterioration of mechanical strength. GB 3 344 624 A patent application discloses a PEEK/PTFE resin layer which is bonded to a backing metal to form a composite bearing. It has been stated that due to the fact that the PTFE particles, present in an amount of 0.1 to 50% by weight, and having a molecular weight of 300,000 to 500,000 could be dispersed in the form of particles in PEEK based resin that the resulting PEEK/PTFE resin layer has greater peeling strength and the bonding force between the backing metal and said resin layer is greater than similar resins with PTFE particles of lower weight.
However it is generally known that carbon fibers (CF) are often added to PAEK/PTFE compositions to improve their mechanical strength and wear resistance such as notably described in JP-A 58-160,346.
JP-A 58-160,346 discloses a PAEK/PTFE/CF resin composition which has outstanding sliding characteristics and comprises a thermoplastic aromatic polyetherketone such as PEEK, as a base material, 10 to 40% by weight of polytetrafluoroethylene which has a mean particle size of less than 20 μm as a fluorocarbon polymer and 10 to 40% by weight of a carbon fiber. It is stated that a high heat distortion temperature (HDT) and a high limiting PV (a product of pressure and velocity) value are obtained by the above resin composition. The thermoplastic aromatic poly(ether ketone) can have an intrinsic viscosity (IV) of between 0.7 and 2.6 (dL/g), measured at 25° C. on a solution of 0.1 g of polymer in 100 ml H2SO4 with a density of 1.84 g/cm3.
This said, it is thus widely recognized that compositions in which PAEK polymers are used as base resin and thereto has added polytetrafluoroethylene polymers and carbon fibers, and the like are used for friction and wear applications. However, it should be said that the limiting PV value is still a medium value thereby restricting its applicability in for example new transmission designs/applications that require the highest friction and wear performance.
There is thus a continuous need for new PAEK/PTFE/CF compositions having improved friction and wear capabilities but also possessing a combination of high performance properties including high mechanical strength, high temperature performance, chemical resistance, and moreover offering superior processability capabilities.